DSP System Implementation of Spread Spectrum Transmission of Multi-channel Measurement Signals

1. Introduction

In the field of measurement and control, it is usually required to transmit multiple detection signals. The transmission of signals is often interfered by the complex environment around them, which will cause large distortion. For example, in the spread spectrum communication transmission system, the spectrum of the transmitted signal is expanded by a pseudo-random sequence in the transmitter and multiplexing of multiple signals is realized by code division multiplexing; it is then despread in the receiver to restore the original transmission signal. The spread spectrum gain of spread spectrum communication can greatly improve the signal-to-noise ratio of the communication system [1-2], increase the reliability of the transmission signal, improve the communication quality, and improve the communication efficiency. At the same time, DSP can meet the requirements of complex algorithm control structure, high computing speed, flexible addressing mode and strong communication performance. The transmission signal parameters can be modified by software, so it has great flexibility. This paper uses DSP system to realize
the spread spectrum transmission of multi-channel measurement signals, combining the advantages of spread spectrum communication and DSP [3-4], which is a promising detection signal transmission implementation method.

2. Spread spectrum transmission system of multi-channel detection signals The system is divided into a transmitting module and a receiving module according to its function. In the transmitting module, multiple baseband digital signals (analog signals are first converted to digital) are spread spectrum modulated by their corresponding pseudo-random sequences. These pseudo-random sequences are different but orthogonal (or quasi-orthogonal) to each other. These sequences are used for spread spectrum modulation and the code division multiplexing technology is used to combine multiple signals into one signal to send it to the main modulator for carrier modulation before it is transmitted. In the receiving module, the received signal is first demodulated for carrier wave, and then correlated with each other using the local pseudo-random sequence that has been synchronized with each channel. Because the pseudo-random sequences corresponding to each channel are independent of each other, the original baseband signal of each channel can be restored. The signal here refers to the digital signal. If an analog signal is required, the digital signal can be converted into an analog signal. This system uses the commonly used sliding correlation capture to synchronize the pseudo-random sequence of the receiving module [5]. Multi-channel measurement

The schematic diagram of the signal spread spectrum transmission system is shown in Figure 1. In the spread spectrum transmission system, the ratio of the spread spectrum signal bandwidth B2 to the information bandwidth B1 is called the processing gain GP, ​​that is,

in spread spectrum communication, after the receiver performs spread spectrum demodulation, it only extracts the signal component of the bandwidth B1 after the spread spectrum sequence correlation processing, and excludes the external interference, noise and other user communication effects extended to the wide band B2, so the spread spectrum processing gain GP can accurately reflect the anti-interference ability of spread spectrum communication.

The larger the code length N of the spread spectrum sequence and the smaller the code element width TC, the larger the code rate Rc and the larger the spread spectrum gain of the spread spectrum communication system.
The higher the spread spectrum processing gain, the stronger the anti-interference ability of the system. Taking the transmission process of a signal with a Gold sequence with a period of 127 as the spread spectrum sequence as an example, the data transmission frequency is 19200, the frequency of the spread spectrum sequence is 19200×127, and the bit error rate is 0.04417 of the non-spread spectrum transmission. The bit error rate when receiving data is reduced by nearly two orders of magnitude.

The Gold spread spectrum sequence used in this system has a period of 127, and its code division multiple access can realize the simultaneous and co-frequency spread spectrum transmission of 12 detection signals. The spread spectrum transmission of multiple detection signals can ensure reliable transmission under the low bit error rate requirements of the receiving end.
3. System structure of DSP implementation of multi-channel detection signal spread spectrum transmission The multi-channel measurement signal spread spectrum transmission system mainly realizes the spread spectrum modulation, synchronization, spread spectrum demodulation and other functions of multi-channel measurement signals (including analog signals and digital signals, analog signals can be converted into digital signals by A/D, digital signals are stored in the system's memory, and then spread spectrum transmission), and is convenient for future expansion to complete other functions. Since this is a DSP hardware platform design, it is guaranteed that the hardware design will not be changed as much as possible or the hardware design will be changed very little in the future function expansion, and only some software needs to be added or modified to achieve its function expansion and upgrade, so the use of dedicated chips is minimized and chips with scalability are used. The overall design block diagram of the whole system is shown in Figure 2.

In the overall design, fixed-point DSP is used to implement the spread spectrum modulation and despreading of multi-channel measurement signals, and FPGA is used to realize the synchronization of spread spectrum signals[7]. The entire system platform includes digital signal processor (DSP) core, FPGA, memory, A/D conversion,
JTAG interface, etc. According to the existing actual situation, the digital signal processor (DSP) uses TI (Texas Instruments) TMS320C5416[6], the FPGA chip uses ALTERA's EP1K100QC208-3, the FLSAH memory uses AMD's AM29LV200, and the A/D conversion uses TI's switch capacitor structure successive approximation type 8-bit A/D converter TLC540. JTAG is the simulation interface connection.

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4. DSP system software design As the control and processing core of the whole system, DSP has to complete a lot of work, which can be summarized as follows:
1. Initialize itself;
2. Load the spread spectrum code sequence and store it in the on-chip RAM, and complete the synchronization of the spread spectrum sequence according to the synchronization signal of FPGA during reception;
3. Receive the data sent by A/D conversion and store it in the pre-opened data interval;
4. Perform spread spectrum modulation on the received multi-channel data respectively, and store the modulated data in the opened data storage interval;
synthesize the multi-channel data after spread spectrum modulation into one channel and perform digital modulation;
perform spread spectrum demodulation on the received spread spectrum signal, restore the original multi-channel signal and send it to the data storage interval.

All DSP software designs of this system are carried out under the CCS2.0 integrated development environment [8], and are written in a mixture of assembly language and C language based on TI's C5000 series DSP. The software flow of its transmitting module and receiving module are shown in Figure 3 (a) and (b) respectively:
This system adopts the method of performing spread spectrum modulation on each measurement signal and then using spread spectrum code code division multiplexing before transmission, without the need for frequency division multiplexing or time division multiplexing before performing spread spectrum modulation for transmission [9]. This makes the system simpler, and can improve the reliability of signal transmission while also improving the system's frequency band utilization. The circuit design mainly involves baseband processing of spread spectrum signals. If wireless spread spectrum transmission of signals is to be realized. Then, on the basis of the design, an RF modulation module can be added, and the baseband signal can be converted into an RF signal after modulation and transmitted. The received RF signal can be demodulated by RF and then despread.
5. Conclusion
In the spread spectrum transmission system of multi-channel measurement signals, different pseudo-random codes are used to modulate different signals to achieve signal multiplexing and spread spectrum transmission. After achieving system synchronization at the receiving end, demodulation is first performed and then despread using the coherent detection method to restore the original signal to realize multi-channel signals. This spread spectrum communication system can realize the effective transmission of multi-channel signals and has the advantages of strong anti-interference ability and easy confidentiality. This system uses the DSP system to realize the spread spectrum transmission of multi-channel measurement signals, making full use of the advantages of DSP devices and the characteristics of the spread spectrum communication system. It is a promising detection signal transmission implementation method.

Figure 3 DSP system software flow
Innovation of this paper: Based on the research on the transmission system of multi-channel measurement signals, this paper proposes a method to expand the spectrum of the transmitted signal and realize the multiplexing transmission of multiple signals by using code division multiplexing. And the multi-channel measurement signal spread spectrum transmission system is realized by using DSP. The experimental results show that the system is feasible.